This invention relates to a mold-press forming apparatus and a formed product manufacturing method for producing a formed product (for example, an optical element such as a high-precision glass lens) by press forming using a mold comprising an upper mold, a lower mold, and a body mold (ex. sleeve) subjected to precision-working in conformity with a desired optical element shape and, in particular, to a mold-press forming apparatus and a formed product manufacturing method capable of mass-producing an optical element high in profile accuracy and surface accuracy with high productivity.
Development has been made of a wide variety of methods for producing an optical element, such as a high-precision glass lens, by press forming using a precision-worked mold without allowing a softened glass to be adhered to the mold (for example, see Japanese Examined Patent Application Publication (JP-B) No. H7-29779 corresponding to U.S. Pat. No. 4,836,838).
The above-mentioned publication discloses an apparatus having a structure in which a formed glass product is produced from a forming material (glass preform) by successively transferring a mold containing the forming material through a plurality of processing chambers including a heating chamber, a pressing chamber, and a cooling chamber sequentially arranged adjacent to each other. By the apparatus having the above-mentioned structure, it is possible to maintain thermal uniformity of the mold and to continuously produce formed products at a high processing speed.
Generally, pressing by a mold-press forming apparatus is carried out at a high temperature. In particular, in case where the forming material is a glass material, a press temperature is as high as 500-800° C. At such a high temperature, pressing must be performed in a nonoxidizing atmosphere in order to protect a mold material (ceramics, cemented carbide, or the like) and a releasing film (containing carbon or precious metal as a main component) formed on a forming surface of the mold.
Accordingly, each processing chamber heated to a high temperature, such as a pressing chamber in which press forming is performed, is evacuated to vacuum by evacuating air or is maintained as filled with a nonoxidizing gas atmosphere such as nitrogen or argon. In order to maintain the atmosphere in each processing chamber, loading and unloading of the mold must be carried out in a state where a gas flow to and from the outside is inhibited.
Referring to FIGS. 1 and 2, description will be made of the structure of the apparatus disclosed in the above-mentioned publication.
The apparatus comprises a loading/unloading chamber P1 disposed above a forming chamber 1, and a plurality of processing chambers (a first heating chamber P2, a second heating chamber P3, a soaking chamber P4, a pressing chamber P5, a first slow cooling chamber P6, a second slow cooling chamber P7, and a rapid cooling chamber P8) which are arranged inside the forming chamber 1 sequentially in a circumferential direction. The inside of the forming chamber 1, i.e., the processing chambers P2 to P8 are continuously under a nonoxidizing gas atmosphere. A mold 4 containing a forming material (later illustrated), which is, for example, glass to be formed, is placed on a support 3 of a rotary table 2 and is successively transferred through the processing chambers P2 to P8. As illustrated in FIG. 1, the processing chambers P2 to P8 are partitioned from one another by a plurality of shutters S1 to S6 (no shutter is formed between the soaking chamber P4 and the pressing chamber P5).
The rotary table 2 has a rotation shaft and an index machine disposed at its center although not shown in the figure. The support 3 has a base 3a provided with a bottom protrusion engaged with a mounting hole 2a formed at an outer peripheral portion of the rotary table 2. The mold 4 containing the glass to be formed is placed on the support 3. The support 3 is disposed so that the mold 4 passes through a substantial center of each of the processing chambers P2 to P8 within the forming chamber 1.
As shown in FIG. 2, a cylinder 5 is disposed below the support 3 in order to move the support 3 upward and downward.
The rotary table 2 is provided with a plurality of mounting holes 2a arranged along a circumferential direction. Each mounting hole 2a is engaged with each of a plurality of supports 3. Thus, a plurality of molds 4 are present in the forming chamber 1 and formed products are continuously formed.
Above the forming chamber 1 and along an axis of a piston rod 5a of the cylinder 5, a seal mount 6 and a bell jar 8 are fixed. The forming chamber 1 is provided with an opening 1a formed on a portion to which the seal mount 6 is fixed. The opening 1a communicates with the seal mount 6. Through the opening 1a, the support 3 moves from the forming chamber 1 into the seal mount 6 and moves out of the seal mount 6 to the forming chamber 1. The seal mount 6 is provided with an evacuating path 7 connected to an evacuating member (will later be described).
The bell jar 8 positioned above the seal mount 6 is driven by a cylinder 9 to be connected to and disconnected from the seal mount 6.
The base 3a of the support 3 is provided with an O ring 3b arranged at a portion covering the opening 1a. The seal mount 6 is provided with an O-ring 6b arranged at a portion kept in contact with an upper surface of the forming chamber 1 and an O-ring 6c arranged at a portion to be brought into contact with the bell jar 8.
A combination of the seal mount 6, the bell jar 8, and the O-rings 3b, 6b, and 6c forms the loading/unloading chamber (airtight chamber) P1.
Next, description will be made of gas exchange at the loading/unloading chamber P1 in the apparatus disclosed in the above-mentioned publication.
The mold 4 is placed on the support 3 and transferred by rotation of the rotary table 2 through the processing chambers P2 to P8. After completion of forming, the mold 4 returns to a position corresponding to the loading/unloading chamber P1. Then, the piston rod 5a is elevated. Following the elevation of the piston rod 5a, the support 3 is elevated also to move the mold 4 through the opening 1a to the seal mount 6.
Consequently, the O-ring 3b arranged at the base (or a flange portion) 3a of the support 3 is pressed against a peripheral edge of the opening 1a to isolate the inside of the forming chamber 1 and the loading/unloading chamber P1 from each other (see two-dot-and-dash lines in FIG. 2). In this state, the bell jar 8 is elevated by the piston rod 9a and the mold 4 containing a formed product is taken out by a robot or the like (not shown). The mold 4 thus taken out is transferred to a step of disassembling the mold 4 by a disassembling device (not shown) to take out the formed product in the mold 4.
After the mold 4 containing the formed product is taken out, a next mold 4 containing a new forming material is placed on the support 3 by a robot or the like (not shown). Next, the bell jar 8 is moved downward until a flange portion 8a of the bell jar 8 is brought into contact with the O-ring 6c of the seal mount 6 to thereby form the loading/unloading chamber P1. The loading/unloading chamber P1 is evacuated first and then filled with a nonoxidizing gas. As the nonoxidizing gas, an atmosphere same as the forming chamber may be used by making the loading/unloading chamber P1 communicate with the inside of the forming chamber.
Then, when the mold 4 is placed on the support 3, the piston rod 5a is moved downward to lower the support 3 and to return the support 3 onto the rotary table 2. In this state, the seal mount 6 and the bell jar 8 are kept in contact with each other. Therefore, even if the opening 1a is opened, the gas within the forming chamber 1 does not leak out.
By the mold-press forming apparatus mentioned above, the formed product is formed in the following manner.
The support 3 with the mold 4 placed thereon is put on the rotary table 2. The mold 4 is transferred by rotation of the rotary table 2 successively through the processing chambers P2 to P8. The mold 4 is at first heated in the heating chambers (the first heating chamber P2, the second heating chamber P3 and/or the soaking chamber P4) to a temperature appropriate for press forming so that the forming material contained therein is softened. With reference to necessary temperature elevation and soaking, a plurality of the heating chambers are used and appropriate temperatures are respectively set. The mold 4 heated to an appropriate temperature is transferred to the pressing chamber P5 having a press shaft.
In the pressing chamber P5, the mold 4 is applied with a predetermined load. Profiles of forming surfaces of the upper and the lower molds precision-worked are transferred and reproduced on the forming material to thereby form the formed product. The formed product is kept contained in the mold 4 and transferred to the cooling chambers (the first slow cooling chamber P6, the second slow cooling chamber P7, and/or the rapid cooling chamber P8) to be cooled to a temperature near a transition point at an appropriate cooling rate.
Thereafter, the mold 4 (containing the formed product) sufficiently cooled is transferred from the cooling chambers to a position corresponding to the loading/unloading chamber P1 and moved to the loading/unloading chamber P1 as described above.
When the mold 4 is moved to the processing chambers P2 to P8, the shutters S1 to S6 between adjacent ones of the processing chambers are opened and closed.
Thus, the mold is successively transferred and press forming is carried out. Simultaneously, at the loading/unloading chamber P1, the mold containing the formed product is taken out and another mold containing a new forming material is introduced. Thus, continuous forming is carried out.
However, the formed product obtained by the continuous forming as described above may suffer occurrence of a defective shape such as uneven thickness. The present inventor found out that the forming material contained in the mold is displaced prior to press forming and a defect such as uneven thickness is caused due to displacement of the forming material.
Specifically, uneven thickness does not occur if press forming is performed in a state where the forming material X is set at the center of the mold, as illustrated in FIG. 3A. On the other hand, if the forming material X contained in the mold is displaced as illustrated in FIG. 3B prior to the press forming, the formed product having uneven thickness is produced during the press forming. This results in a defective shape and insufficiency of an effective optical diameter of an optical element as the formed product.
When the forming material X is contained in the mold, the forming material X is kept in contact with forming surfaces of the upper and the lower molds and lightly clamped therebetween. Therefore, once the forming material X is displaced and decentered, the forming material X hardly returns to an initial position (center position).
In view of the above, position control upon positioning the forming material in the mold is sufficiently carried out so as to position the forming material at the center of the mold. However, even if the forming material is placed at the center of the mold, the above-mentioned uneven thickness is caused to occur.
Under the circumstances, the present inventor investigated the cause of the above-mentioned displacement and analyzed the behavior of the mold in the loading/unloading chamber P1 upon gas exchange at the loading/unloading chamber P1. As a result, it has been found out that the forming material in the mold is moved due to rapid evacuation from the loading/unloading chamber or that the mold itself vibrates to cause displacement of the forming material contained therein. Further, it has been found out that the mold is inclined on the support due to evacuation to cause displacement of the forming material.
In particular, the above-mentioned displacement is remarkable in case where the forming material has a spherical shape or a radius of curvature of a concave forming surface of the lower mold is relatively large.